Sap flow as a function of variables within nested scales: ordinary least squares vs. spatial regression models

Abstract

Understanding scale-dependent influential drivers of sap flow variability can help managers and policymakers to allocate resources within a particular scale to improve forest health and resiliency against water-stress stimuli such as drought and insects, e.g., bark beetle infestations. We defined a daily measure of sap flow as a function of variables within nested scales of landscape, stand, and tree, using ordinary least squares (OLS), spatial lag and error regression models. Model covariates were elevation, latitude (Y-coordinate), longitude (X-coordinate), neighborhood tree density, tree diameter at breast height (DBH), and bark temperature for 40-surveyed Norway spruce (Picea abies) in the Czech Republic, Central Europe. Trees were spatially distributed within 19-established subplots across five plots, with distances ranging 2 - 9 km, at which variations in soil water potential and temperature were limited. The daily measure of sap flow within the regional scale allowed us to avoid the temporal and spatial variability of climate effects on sap flow. A relatively flat terrain across subplots also allowed us to control the effects of slope, aspect, and topography-related solar incidence angle on sap flow. Sap flow was strongly spatially autocorrelated, so OLS models failed to take spatial autocorrelation into account unless to some extent, depending on the spatial distribution of samples, by including latitude and/or longitude in the models. Among spatial regression models, spatial error models performed better than lag models, allowing to capture the effects of unmeasured independent variables. Sap flow variability for the most part (~70%) was explained by the landscape-level variable of elevation followed by the stand-level variable of tree density, and the remaining part by variables related to tree characteristics; a nested down-scaling function, defined and visualized for the first time. Therefore, thinning forest stands and future plantations with optimum distances, based on the elevation gradients, may be required to counterbalance the allocation of resources, e.g., water, nutrients, and light, among trees, leading to enhance forest health and resiliency against water-stress stimuli.

Effect of diurnal solar radiation regime and tree density on sap flow of Norway spruce (Picea abies [L.] Karst.) in fragmented stands

“…These results build on the existing evidence of competition effects by neighboring trees (Canham, 2004;Bhandari et al, 2021;Zabihi et al, 2023). As sessile organisms, trees cannot relocate to more favorable environments when they encounter stressful conditions.…”

Understanding bark beetle outbreaks: exploring the impact of changing temperature regimes, droughts, forest structure, and prospects for future forest pest management

“…Likewise, highly dense plantations, often driven by commercial goals, exhibit poor growth and resilience to stress. Increased tree density can increase intraspecific competition among individual trees and constrain resource availability (Zabihi et al 2023;Thomas et al 2024). Research indicates that increased competition within a forest stand can exacerbate the impacts of drought and increase the likelihood of mortality, particularly if water is the limiting resource (Zhang et al 2015;Young et al 2017;Korolyova et al 2022).…”

“…Forest structures vary across elevation, slope, and aspect gradients, with elevation playing a crucial role in tree mortality during all phases of I. typographus infestation (Mezei et al 2014a;Sproull et al 2015). For example, trees tend to be more densely packed at lower elevations within a limited altitude range (Mazón et al 2020;Zabihi et al 2023). This difference in tree density due to elevation also affects tree characteristics like bark temperature and diameter (Fig.…”

“…This difference in tree density due to elevation also affects tree characteristics like bark temperature and diameter (Fig. 2; Zabihi et al 2023) which are important predictors of host susceptibility to bark beetles. Since topography and inter-tree spacing significantly modulate ecosystem functions, forest management strategies should be tailored considering these variables to foster improved overall health and resilience of secondary spruce forests (Zabihi et al 2023;Thomas et al 2024).…”

An Optimized and Cost-Effective RNA Extraction Method for Secondary Metabolite-Enriched Tissues of Norway Spruce (Picea abies)

“…Ongoing climate change has had a negative impact on the Norway spruce population in the forest [1]. The rise in spruce mortality over the past few decades can be attributed to a combination of biotic factors, such as bark beetle (Ips typographus) outbreaks and fungal pathogens, and abiotic factors, including drought and windthrows, or a combination of both [2][3][4]. In particular, bark beetle outbreaks are often triggered by environmental factors, such as droughts and increased temperatures, resulting in compromised tree defense.…”